Method of forming an integrated circuit device package using a temporary substrate

ABSTRACT

Abstract of Disclosure 
     Methods for forming packages for housing an integrated circuit device are disclosed.  In one embodiment, a plastic sheet having an first surface is provided.  An array of package sites is formed on the first surface of the plastic sheet.  The package sites each include a metal die pad and leads surrounding the die pad.  The package sites may be formed by applying a first metal layer on the first surface of the plastic sheet, and then applying a second metal layer in a pattern that defines the die pads and leads of the package sites.  The first metal layer is then selectively etched using the second metal layer as an etch mask.  Next, an integrated circuit die is placed on each die pad of the array.  The die is electrically connected to the leads surrounding the respective die pad.  An encapsulant is applied onto the first surface of the plastic sheets so as to cover the package sites.  The plastic layer is removed by peeling, by dissolving the plastic sheet, or dissolving an adhesive connection between the plastic sheet and the encapsulated array.  The packages are then singulated from the array.

Cross Reference to Related Applications

[0001] This application is a continuation-in-part of U.S. patent application No. 09/383,022 (attorney docket M-5311 US), which is entitled "Method of Forming an Integrated Circuit Package Using Plastic Tape as a Base" and was filed on August 25, 1999.

Background of Invention

[0002] A problem with conventional plastic packages is that their internal leadframes limit reduction of the size of the packages. Practitioners have attempted to reduce the size of packages by eliminating internal leadframes, as is shown in U.S. Patent No. 4,530,142 to Roche et al.

[0003] Roche et al. begins with a metal temporary substrate. A layer of a low melting-point alloy is applied onto to the metal temporary substrate. Next a plurality of metal die pads and leads are formed on the low-melting point alloy layer. An integrated circuit device is placed on each of the die pads and connected to the leads surrounding the respective die pad. The integrated circuit devices are then encapsulated in a single block of encapsulant material. Individual packages are then cut from the block of hardened encapsulant.

[0004] The methods and package of Roche et al. have foreseeable disadvantages. For example, the use of the metal temporary substrate and low-melting point alloy layer increase costs and manufacturing difficulty. Further, the packages are believed to be unreliable because the contacts could easily be pulled from the encapsulant material.

[0005] A package marketed by Toshiba Corporation of Japan under the name "BCC" is believed to be made as follows. A copper sheet is partially etched through in certain locations, forming pockets isolated by unetched copper. A central pocket is surrounded by several smaller satellite pockets. The copper sheet is then masked, leaving the pockets exposed. Next, the pockets are plated with layers of gold, nickel, and gold. An integrated circuit device is placed in the central pocket. (In some embodiments, there is no central pocket, so the device is simply placed on the copper sheet.) Bond wires are connected between the device and the satellite pockets. Next, the device and bond wires are encapsulated. Finally, the remainder of the copper sheet is etched away by acid, forming a completed package. Once the copper is removed, the metal plated into the satellite pockets forms the leads of the package, and the metal plated into the central depression (if any) is the die pad.

[0006] This process is believed to have several disadvantages. First, the use of acid to dissolve the remainder of the copper plate after encapsulation creates a significant possibility of contamination, since such acids are generally regarded as dirty. Second, the package is subject to failure, because the leads are attached to the package only by the bond wire and by the adhesiveness of the encapsulant to the inner surface of the plated pocket. Thus, the leads could easily be detached from the bond wire and package body. Third, epoxy encapsulant material sometimes does not adhere well to gold.

[0007] Accordingly, there is a need for a small and reliable package that is easier and less expensive to manufacture than prior art packages.

Summary of Invention

[0008] The present invention includes a method of manufacturing a package for housing an integrated circuit device. In one exemplary embodiment, Step 1 provides a plastic sheet having an adhesive first surface. The plastic sheet may be plastic tape. Step 2 attaches a metal sheet onto the first surface of the plastic sheet, and then forms an array of package sites by selectively removing portions of the metal sheet. Each package site includes a die pad and a plurality of satellite leads around the die pad. Step 3 attaches an integrated circuit device to each of the die pads. Step 4 connects a conductor, such a bond wire, between each of a plurality of conductive pads on the integrated circuit device and one of the leads of the respective package site. Step 5 applies an encapsulating material onto the first surface of the plastic sheets, integrated circuit devices, the leads, and the electrical conductors of each package site. Step 6 hardens the encapsulating material. Step 7 removes the plastic sheet. Optional Step 8 applies solder balls to the exposed surfaces of the leads of the package sites. Finally, Step 9 separates individual packages from the encapsulated array. An alternative embodiment in an LCC style package requires no solder balls.

[0009] An embodiment encompassed within the present invention includes forming a reentrant portion (or reentrant portions) and aspirates on the side surfaces of the die pads and leads of the package sites. During the encapsulation step, the encapsulant material flows into the reentrant portions and aspirates. The reentrant portions and aspirates engage the encapsulant material and lock the die pad and leads to the encapsulant material of the package.

[0010] The present invention overcomes the disadvantages of the prior art by, among other things, the use of an inexpensive plastic sheet as a base for forming the packages, and by the formation of encapsulant locking features on the side surfaces of the die pad and leads. These and other advantages will become clear through the following detailed description.

Brief Description of Drawings

[0011]Figure 1 is a flow chart of a method of forming a package for an integrated circuit device.

[0012]Figure 2 is cross-sectional side view of a plated metal sheet on a plastic sheet.

[0013]Figure 3 is a cross-sectional side view of an array of package sites each including a die pad and leads.

[0014]Figure 4 is a top plan view of the array of package sites shown in Figure 3.

[0015]Figure 5 is a cross-sectional side view of these integrated circuit devices placed on and wire-bonded to leads of respective package sites.

[0016]Figure 6 is a cross-sectional side view of an array of package sites after encapsulation by either liquid encapsulation or molding techniques.

[0017]Figure 7 is a cross-sectional side view of the array of Figure 5 after removal of the plastic sheet and attachment of solder balls to the leads.

[0018]Figure 8 is a cross-sectional side view an inverted array of packages after the encapsulant material is cut with a saw.

[0019]Figure 9 is a cross-sectional side view of a completed package.

[0020]Figure 10 is a top plan view of an array of package sites with a bead of adhesive material around the array.

[0021]Figure 11 is a cross-sectional side view of a completed package showing a first alternative side surface of a die pad and leads.

[0022]Figure 12 is a cross-sectional side view of an array of individually molded package bodies.

[0023]Figure 13 is a cross-sectional side view of a completed package showing a second alternative side surface of a die pad and leads.

[0024]Fig. 14 is a plan view of a package site having two rows of staggered leads around the die pad.

[0025]Fig. 15 is a flow chart of an alternative method of making a package.

[0026]Fig. 16 is a cross-sectional side view of a plastic sheet having a first metal layer and a second metal layer thereon that serves as a mask for etching the first metal layer.

[0027]Fig. 17 is a cross-sectional side view of the die pad and leads of a package site formed by the method of Fig. 14.

[0028]Fig. 18 is a flow chart of an alternative method of forming a package.

[0029]Fig. 19 is a cross-sectional side view of a plastic sheet with two layers of an electrically-conductive media thereon, where the layers define an array of package sites.

Detailed Description

[0030]Figure 1 is a flow chart of an exemplary method 5 in accordance with the present invention for forming a package for an integrated circuit device. An example of a completed package 37 that may be formed by method 5 of Figure 1 is shown in Figure 9.

[0031] As will become clear from the discussion the assembly method of Figure 1, package 37 of Figure 9 includes a semiconductor integrated circuit device 28 mounted on a metal die pad 20. A plurality of metal leads 24 are adjacent to die pad 20 and are connected by bond wires 29 to conductive pads (not shown) on device 28. Encapsulant material 32 forms the package body, and covers device 28, bond wires 29, and the upper and side surfaces of die pad 20 and leads 24. The lower surface of die pad 20 is exposed. Solder balls 34 are attached to the lower surface of leads 24, although solder balls are optional.

[0032] Again, Figure 1 provides a method 5 of making a package like package 37 of Figure 9. Referring to Figures 1 and 2, Step 1 of Figure 1 provides a plastic sheet formed of a plastic material. Plastic sheet 10 has a first surface 11 and an opposite second surface 12. An adhesive is present on or applied to first surface 11 of plastic sheet 10.

[0033] Plastic sheet 10 may be a segment having a size of, for example, 5 cm by 20 cm, and a thickness of about 25 to 75 microns. The size and thickness of plastic sheet 10 can vary. Alternatively, plastic sheet 10 may be a plastic tape that is on a roll. As described below, the process of Figure 1 may be performed as a reel to reel process.

[0034] Plastic sheet 10 is formed of a conventional plastic material such as polyvinyl chloride, polyvinyl alcohol acetate, polypropylene acetate, vinyl, polyethylene, mylar, or polyimide. An example brand of plastic tape is KAPTON polyimide tape from the Dupont Company.

[0035] Step 2 of Figure 1 provides a patterned metal sheet on the first surface of the plastic sheet. Figures 2 and 3 show one embodiment of a method of performing Step 2 of Figure 1.

[0036] Referring to Figure 2, a metal sheet 13 is first attached to the adhesive first surface 11 of plastic sheet 10. Metal sheet 13 of Figure 2 has a first surface 14 attached to first surface 11 of plastic sheet 10, and an opposite second surface 15. In an alternative embodiment, an adhesive material is applied to first surface 14 of metal sheet 13 rather than to first surface 11 of plastic sheet 10.

[0037] In Figure 2, metal sheet 13 consists of an underlying first metal layer 16 on first surface 11 of plastic sheet 10, and a second metal layer 17 that is plated onto a top surface of first metal layer 16 opposite first surface 11 of plastic sheet 10. First metal layer 16 of metal sheet 13 may be formed, for example, of copper, a copper alloy, or Alloy 42. Plated metal layer 17 may be nickel gold, silver, platinum, palladium, or another noble metal. Using nickel/gold plating or silver plating may facilitate bond wire connections. Where copper is used for metal sheet 13, without plating, copper bond wires may be used. Although not shown in Figure 2, first surface 14 of metal sheet 13 also may be plated. Metal sheet 13 may have a thickness of, for example, 100 to 250 microns.

[0038] Next, metal sheet 13 is etched to form an array of package sites. For example, such etching is performed by chemical etching using a patterned photoresist mask. In such a process, a layer of photoresist is applied onto second surface 15 of metal sheet 13. Next, the photoresist is exposed to light through a patterned reticle or the like, and is subsequently developed to form a mask. Chemicals are sprayed or otherwise applied to the masked metal strip, and exposed portions of metal are etched away, leaving the desired pattern. Typically, acids are used to etch the metal. Obviously, plastic film 10 must not be substantially attacked by the chemical used to etch metal sheet 13. A cleaning step may be necessary to remove residue or other undesirable materials after Step 2 (or any of the other steps) of Figure 1.

[0039]Figure 3 is a cross-sectional side view of an array 18 of three identical package sites 19 formed by the above-described chemical etching of metal sheet 13. Figure 4 is a top plan view of array 18 of Figure 3.

[0040] Referring to Figures 3 and 4, each package site 19 includes a metal die pad 20 and metal leads 24. Die pad 20 and leads 24 are formed from metal layer 16 and metal layer 17 of metal sheet 13 of Figure 2. Figure 4 shows that each die pad 20 has a rectangular perimeter and is surrounded on each of its four sides by four leads 24. Leads 24 also have a rectangular perimeter.

[0041] The perimeter shapes of die pad 20 and leads 24 can vary. For example, leads 24 can have a rectangular or circular perimeter. Also, the number and positioning of the leads can vary. For example, 64 or 128 leads may be selected. As another example, instead of having single rows of leads 24 around die pad 20, two or more staggered rows of leads 24 may be provided adjacent to two or all four sides of die pad 20. Fig. 14 is a plan view of an exemplary embodiment of a package site 19 where two staggered rows of leads 24 surround die pad 20.

[0042] In Figure 3, each die pad 20 has a first surface 21 attached to first surface 11 of plastic sheet 10, an opposite second surface 22, and side surfaces 23 at the periphery of die pad 20 between first surface 21 and second surface 22. Each lead 24 has a first surface 25 attached to first surface 11 of plastic sheet 10, an opposite second surface 26, and side surfaces 27 at the periphery of lead 24 between first surface 25 and second surface 26.

[0043] Step 3 of Figure 1 places an integrated circuit device onto each of the die pads. Step 4 of Figure 1 connects a conductor, such a bond wire, between each of a plurality of conductive pads on each of the integrated circuit device and one of the leads of that integrated circuit device's respective package site.

[0044]Figure 5 is a cross-sectional side view of three integrated circuit devices 28 placed on second surface 22 of these die pads 20. Each integrated circuit device 28 is placed on a die pad 20 and is adhesively attached to second surface 22 of die pad 20 using conventional die attach equipment and adhesives.

[0045] Each integrated circuit device 28 of Figure 5 includes conductive pads (not shown) that are connected to internal circuitry of the integrated circuit device. Each conductive pad is connected by a conventional metal bond wire 29 to a second surface 26 of a lead 24. Conventional bond wire equipment is used.

[0046] Step 5 of Figure 1 applies an insulative and adhesive encapsulating material onto the array of the package sites in order to encapsulate each of the integrated circuit devices, the leads, and the electrical conductors of each package site. Step 6 of Figure 1 hardens the encapsulating material forming the package bodies and the exterior surfaces of the packages.

[0047]Figure 6 is a cross-sectional side view of three package sites 19 of an encapsulated array 18 after Steps 5 and 6. The three package sites 19 are covered in a single block of encapsulant material 32. In particular, first surface 11 of sheet 10 and each integrated circuit device 28, die pad 20, leads 24, and bond wires 29 are covered by encapsulating material 32. Second surface 22 and side surfaces 23 of die pad 20, and second surface 26 and side surfaces 27 of leads 24 are covered by encapsulant material 32. Encapsulant material 32 does not, however, cover second surface 12 of plastic sheet 10, and plastic sheet 10 prevents encapsulant material from covering first surface 21 of die pad 20 and first surface 26 of leads 24.

[0048] Although in this embodiment three packages sites 19 are encapsulated in a single block of encapsulating material 32, the package sites may be encapsulated individually in an alternative embodiment of the process. In such an alternative embodiment, individual package sites may be encapsulated by injection or transfer molding, among other possible methods.

[0049] Steps 5 and 6 of Figure 1 may be performed in several alternative ways. Peripheral side surface 30 of encapsulated array 18 is orthogonal, to illustrate a liquid encapsulation process. Peripheral side surface 31 is sloped to illustrate a molding process for encapsulating array 18.

[0050] For example, Step 5 may be performed using a liquid encapsulant. In such a method, a first step applies a contiguous bead of a conventional hardenable viscous adhesive material onto first surface 11 of plastic sheet 10 around one of more package sites 19. An example bead material is HYSOL 4451 epoxy from the Dexter-Hysol Company of City of Industry, California. Figure 10 is a top plan view of an array 18 of three package sites 19 with a bead 33 of adhesive material around the array 18. Bead 33 is cross-hatched in Figure 10. Bead 33 encloses each package site 19. Bead 33 forms a cavity with first surface 11 of plastic sheet 10 in which the three package sites 19 and integrated circuit devices 28 are enclosed. Next, bead 33 is solidified, such as by heating at 150 degrees C for one hour. Next, a conventional, hardenable, adhesive, and insulative liquid encapsulating material suitable for encapsulating packages is poured within bead 33 and fills the cavity formed by bead 33 so that package sites 19, integrated circuit devices 28, leads 24, die pads 20, and bond wires 29 are covered with encapsulant material. An example liquid encapsulant material is HYSOL 4450 encapsulant. As a final step, the encapsulant material is hardened, such as by heating at 150 degrees C for one hour. This embodiment of Steps 5 and 6 forms a single solid block of encapsulant material 32 above and on array 18.

[0051] Alternatively, Steps 5 and 6 of Figure 1 may be accomplished using conventional plastic molding techniques and materials, such as injection molding or transfer molding. In such a method, a first step places array 18 in a conventional two-pocket mold. The lower pocket of the mold is blanked out by a bar so that encapsulant material does not enter the lower pocket. Next, insulative encapsulant material, i.e., molding compound, is provided to the upper pocket of the mold. Encapsulant material 32 is molded onto package sites 19 of array 18 above and on first surface 11 of plastic sheet 10. Integrated circuit devices 28, leads 24, die pads 20, and bond wires 29 are covered with encapsulant material 32. Next, encapsulant material 32 is hardened in a conventional manner. Where injection molding techniques are used, example encapsulation materials include styrene, liquid crystal polymer, or nylon 66. Where transfer molding techniques are used, SUMITOMO 8100 molding material from the Sumitomo Company of Japan or Plaskon SMT B1RC molding material may be used.

[0052] Step 7 of Figure 1 removes plastic sheet 10 from encapsulated array 18. An example method of performing Step 7 is to dissolve plastic sheet 10 in a solvent, such as acetone. The material of plastic sheet 10 should be chosen in view of its ability to be dissolved, and the solvent must be compatible with the encapsulant material 32. Another method of performing Step 7 is simply to use a solvent to dissolve the adhesive that was used to attach plastic sheet 10 to metal sheet 13 without dissolving plastic sheet 10. Plastic sheet 10 may then fall away from the encapsulated array or may be peeled away. Still another method of performing step 7, where plastic sheet 10 is polyimide, for example, is to soak array 18 in heated water (e.g., 80 degrees C water) for about an hour, then apply ultra violent light to sheet 10 until sheet 10 falls off or may easily be peeled off array 18. For example, ultraviolet light may be applied for about one minute.

[0053] Another method of performing Step 7 is to heat plastic sheet 10 and then peel plastic sheet 10 from array 18. For example, the heating could be to a temperature of 80 degrees C. If Step 5 is done by molding, then plastic sheet 10 could be removed while array 18 is in the mold or after array 18 is removed from the mold.

[0054] Step 8 is an optional step that applies conventional solder balls to the exposed surfaces of the leads of encapsulated package sites 19. Figure 7 is a cross-sectional side view of array 18 of packages sites 19 after removal of plastic sheet 10 and attachment of solder balls 34 to the exposed first surfaces 25 of leads 24. Solder balls 34 are used for connection of the package to external circuitry.

[0055] Alternatively, Step 8 may be omitted. In such a case, first surface 25 of leads 24 serve as connectors to external circuitry, as in a leadless chip carrier package style.

[0056] Finally, Step 9 of Figure 1 separates individual packages from the encapsulated array. Step 9 may be performed, for example, by cutting encapsulated array with a saw. Disposable material, such as bead 33 is cut away.

[0057]Figure 8 is a cross-sectional side view of array 18 of Figure 7 after encapsulant material 32 is cut with a saw. Encapsulated array 18 is inverted and orthogonal cuts 35 in encapsulant material 32 are made. Prior to cutting, an adhesive plastic film 36 is applied to the top surface of encapsulated array 18 to immobilize the packages during the cutting step. Such films are used, for example, in conventional wafer cutting processes. Adhesive plastic film 36 is placed on the top surface of encapsulated array 18 before Step 7, Step 8, or Step 9. The cutting step segments encapsulant material 32 without fully severing plastic film 36. The cuts segment the encapsulant material without fully severing the tape. When adhesive film 36 is removed from the package tops, the formation of individual packages 37 is completed. Figure 9 shows a completed package 37 having orthogonal side surfaces 38 at the periphery of package 37.

[0058] Another possible technique for singulating packages 37 (Fig. 9) from the encapsulated array 18 of package sites 19 (Fig. 7) does not require the use of a plastic film 36 (Fig. 8). In this alternative method, the encapsulated array 18 is placed on a vacuum chuck formed, for example, of fritted metal. The chuck has criss-crossing grooves in the X and Y directions. The grooves are, for example, 0.05 to 0.08 mm in depth. The grooves correspond to where cuts are to be made between package sites 19 of the encapsulated array 18. A saw cuts along the grooves without contacting the chuck to singulate the packages. A water spray may be used for cooling and/or removal of waste. The packages are then removed from the chuck.

[0059] As mentioned above, plastic sheet 10 may be a continuous tape on a reel. Where plastic sheet 10 is a continuous tape on a reel, then the process of Figure 1 can be a reel to reel process. For example, Step 1 of Figure 1 may include unrolling a plastic tape from a first reel, attaching a metal tape to an adhesive surface of the unrolled plastic tape, and then rolling up the joined plastic and metal tapes on a second reel. Step 2 may include unrolling the second reel of joined plastic and metal tapes, patterning a selected length of the metal to form package sites 19, and then rolling the etched length of package sites 19 onto a third reel. The second reel is advanced until the entire length of the second reel is etched to form a third reel of package sites 19. The third reel of package sites 19 is then unrolled, and segments of selected length, for example, from four to 200 package sites 19, are subjected to die attach, bond wire attach, and encapsulation (i.e., Steps 3-6 of Figure 1). After Step 6 (encapsulant hardening), the selected length of encapsulated package sites 18 can be cut from the third reel, and processed as an individual encapsulated array 18 through the remaining steps of Figure 1.

[0060] Alternatively, if, for example, Step 5 of Figure 1 is performed by molding, then a mold having multiple two-pocket cavities can be used which molds individual package housings on a selected number of package sites 19. The lower pocket is blocked with a bar so encapsulant material only is applied above first surface 11 of plastic sheet 10. This is shown in Figure 12, where three package sites 19 are molded into individual package bodies 39. Molded webbing 40 is between the individual package. In such a process, after Step 6, the reel of molded package sites is advanced onto a fourth reel, and a second segment of packages is molded. The third reel of package sites is then advanced until all of the package sites are subjected to die attach, wire bond, and molding. Next, the fourth roll of molded packages is unrolled, and Steps 7-9 of Figure 1 are performed as discussed above.

[0061] In package 37 of Figure 9, side surfaces 23 of die pad 20 and side surfaces 27 of leads 24 are shown as having an orthogonal orientation. Alternatively, side surfaces 23 and 27 are formed with a reentrant portion for enhancing the connection between encapsulant material 32 and die pad 20 and leads 24. Such reentrant surfaces may be formed, for example, by a controlled chemical etch process, or by a standard chemical etch process followed by coining of the patterned metal sheet.

[0062]Figure 11 shows an enlarged cross-sectional side view of a completed package 50 having alternative side surfaces 23 of a die pad 20 and side surfaces 27 of leads. The reentrant portions of side surfaces 23 and 27 are formed during Step 2 of Figure 1, when patterned array 18 is formed from plated metal sheet 13. Package 50 is the same as package 37 of Figure 9, except for the side surfaces of the die pad and leads.

[0063] In Figure 11, the reentrant portion of side surfaces 23 of die pad 20 and side surfaces 27 of leads 24 includes a central depression 42. In addition, side surfaces 23 and 27 have a roughly-textured surface that includes numerous aspirates on the reentrant surface. Encapsulant material flows into central depression 42 and into the areas of the aspirates during Step 5 of Figure 1. The reentrant portion and the aspirates engage encapsulant material 32 and lock die pad 20 and leads 24 to encapsulant material 32.

[0064] Referring to Figure 11, reentrant side surfaces 23 of die pad 20 and side surfaces 27 of leads 24 can be formed during step 2 by etching metal sheet 13 with a conventional liquid etchant using a controlled etch process. The etch process is continued beyond the time that would be required to form orthogonal side surfaces for the die pad and leads. This is usually accomplished by using an oversized mask on upper second surface 15 of metal sheet 13 (Figure 2) and using a slight overetch. The size and shape of depression 42 of Figure 11 is controlled by the amount of over-etching.

[0065]Figure 13 shows a second alternative package 60 that may be formed by the method of Figure 1. Package 60 is identical to package 50 of Figure 11, except that the side surfaces of die pad 20 and leads 24 have a different reentrant profile. Referring to Figure 13, side surfaces 23 of die pad 20 have a projecting lip 61 adjacent to upper second surface 22. Lip 61 includes a roughly textured surface with aspirates. Side surface 23 of die pad 20 has a reentrant orthogonal portion 62 beneath lip 61, i.e., between lip 61 and lower first surface 21. Leads 24 also have a similar projecting lip 64 with aspirates. Side surfaces 27 of leads 24 also have a reentrant orthogonal portion 65 beneath lip 64, i.e., between lip 64 and lower first surface 25. During Step 5 of the above described process, encapsulant material 32 covers lips 61 and 64, and flows beneath lips 61 and 64 to contact reentrant orthogonal portions 62 and 65. The encapsulant material beneath lips 61 and 64 lock die pad 20 and leads 24 to encapsulant material 32.

[0066] The reentrant side surfaces of die pad 20 and leads 24 of Figure 13 are formed during Step 2 of the process of Figure 1. In particular, a first step provides a patterned metal sheet having orthogonal side surfaces on die pad 20 and leads 24, as would be produced in a standard chemical etching process. A second step coins upper second surface 22 of die pad 20 and upper second surface 26 of leads 24. Coining involves applying a high pressure impact to upper second surfaces 22 and 26. This high pressure impact deforms the side surfaces of die pad 20 and leads 24 to form lips 61 and 64 (Figure 13).

[0067]Fig. 15 is a flow chart of another method 70 within the present invention of forming a package. Referring to Fig. 16, Step 1 of method 70 provides a plastic sheet 10. Step 2 applies a metal layer 16 to a first surface 11 of plastic sheet 10. Metal layer 16 may be applied in a variety of ways. For example, metal layer 16 may be applied by sputtering or chemical vapor deposition. Alternatively, metal layer 16 may be a metal sheet that is attached to first surface 11 using an adhesive present either on the metal sheet or on first surface 11. For the purpose of example, assume that metal layer 16 is copper plated with nickel, although other types of metal may be used.

[0068] Step 3 applies a photoresist pattern to the exposed second surface 15 of metal layer 16. The photoresist pattern is used to define a plurality of package sites 19, each including a die pad 20 and adjacent leads 24. Step 4 applies a second metal layer 17 on the exposed areas of metal layer 16. As an example, assume that second metal layer 17 is gold and that the gold is plated on the nickel layer, which in turn is plated on the copper layer. Next, the photoresist pattern is stripped away, leaving gold areas where die pads 20 and leads 24 will be formed, as shown in Fig. 16.

[0069] Step 5 selectively etches the first metal layer 16 using second metal layer 17 as an etching mask. In this example, the chemical selected as the etchant, typically an acid, must etch copper and nickel without etching or substantially etching the gold. An array of package sites 19 is thus formed, as shown in Fig. 3.

[0070]Figure 17 is a cross-sectional side view of a die pad 20 and two leads 24 of a package site 19 produced by Step 5 of method 70 using the exemplary metals discussed above. Die pad 20 and leads 24 include a first layer of copper 14, an intermediate layer of nickel 14a and a top layer of gold 17. The side surfaces 23 and 27 of die pad 20 and leads 24, respectively, have reentrant portions in copper layer 14 and nickel layer 14a. The reentrant portions of side surfaces 23 and 27 fill with encapsulant during a subsequent encapsulation step, and thereby lock die pad 20 and leads 24 to the encapsulant.

[0071] Steps 6-12 of method 70 are similar to Steps 3-9 of method 5 of Fig. 1, and thus do not require further discussion. As with method 5, method 70 may be performed as a reel-to-reel process.

[0072]Figure 18 is a flow chart of an alternative method 80 within the present invention of making a package. Step 1 of method 80 provides a plastic sheet 10, which may be, for example, polyimide. Alternatively, plastic sheet 10 may be polyester or polyethermide. Plastic sheet 10 may have an adhesive layer on first surface 11 thereof, but an adhesive layer is not necessary. Referring to Fig. 19, Step 2 applies an electrically conductive, metal-containing first media 81 onto first surface 11 of plastic sheet 10 in a pattern using a silk screening method. The pattern defines an array 18 of package sites 19, each having a die pad 20 and leads 24. Step 3 applies an optional second electrically conductive, metal-containing second media 82 onto first media 81 using a silk screening method. In this embodiment, second media 82 is applied so as to overhang all of the peripheral edges of media 81 so as to form die pads 20 and leads 24 with side surfaces 23 and 27 having a reentrant profile that can lock die pads 20 and leads 24 to the encapsulant. The silk screening may be performed through a 1000 mesh stainless steel screen.

[0073] First media 81 and second media 82 of Fig. 19 may be low temperature, adhesive, metal-containing materials. For example, epoxy-based media or ink-based media may be used. The metals contained therein may vary. For example, first media 81 and second media 82 may contain copper and silver, respectively. Gold or aluminum containing media also may be used. A low temperature media is used to avoid melting plastic sheet 10. Aluminum bond wires may be used.

[0074] In an alternative embodiment, a stencil method is used instead of a silk screening method to form package sites 19.

[0075] Steps 4-10 of method 80 are similar to the steps 3-9 of method 5 of Fig. 1, and thus do not require detailed discussion. With respect to Step 8 of method 80, plastic sheet 10 may be removed from array 18 by any of the methods discussed above. For example, plastic sheet 10 may be heated and peeled from array 17. Alternatively, array 18 may be placed in a solvent that either dissolves an adhesive connection between array 18 and plastic sheet 10, or dissolves plastic sheet 10 itself. As with method 5 of Fig. 1, method 80 of Fig. 18 may be performed as a reel-to-reel process.

[0076] The embodiments described herein are merely examples of the present invention. Artisans will appreciate that variations are possible within the scope of the claims. 

Claims 1.A method of forming a package for an integrated circuit device comprising: providing a plastic sheet having a first surface; applying a first metal layer to the first surface of the plastic sheet; patterning the first metal layer so as to form an array of package sites, wherein each package site formed includes a die pad and a plurality of leads adjacent to the die pad; placing an integrated circuit device on each die pad of the array; electrically connecting each integrated circuit device to at least some of the leads adjacent to the die pad on which the integrated circuit device is placed; applying an encapsulating material onto the array and the first surface of the plastic sheet so that each package site is encapsulated; removing the plastic sheet from the array; and separating individual packages from the encapsulated array, wherein a planar first surface of the die pad and leads of the packages is exposed at an external surface of each package. 2.The method of claim 1, wherein removing the plastic sheet comprises using a solvent to dissolve a connection between the array and the plastic sheet so that the plastic sheet may be separated from the array. 3.The method of claim 2, wherein an adhesive material is applied to the first surface of the plastic sheet prior to application of the metal layer. 4.The method of claim 1, wherein applying the first metal layer to the first surface of the plastic sheet comprises: providing a sheet of metal: and attaching the sheet of metal to the first surface of the plastic sheet with an adhesive. 5.The method of claim 4, wherein removing the plastic sheet comprises using a solvent to dissolve a connection between the array and the plastic sheet so that the plastic sheet may be separated from the array. 6.The method of claim 1, wherein the first metal layer comprises a layer of one type of metal plated with one or more layers of another type of metal. 7.The method of claim 1, wherein patterning the metal layer comprises: applying a second metal layer in a pattern on the first metal layer, wherein the pattern of the second metal layer defines said package sites; and selectively etching the first metal layer to form said package sites using said second metal layer as a mask. 8.The method of claim 7, wherein the first metal layer comprises copper and the second metal layer is gold. 9.The method of claim 8, wherein the first metal layer comprises a layer of copper and a layer of nickel, wherein said gold is applied onto said layer of nickel. 10.A method of forming a package for an integrated circuit device comprising: providing a substrate sheet having a first surface; applying a first layer of an electrically conductive material on the substrate sheet, wherein said first layer is applied in a pattern that defines an array of package sites, wherein each package site formed includes a die pad and a plurality of leads adjacent to the die pad; placing an integrated circuit device on each die pad of the array; electrically connecting each integrated circuit device to at least some of the leads adjacent to the die pad on which the integrated circuit device is placed; applying an encapsulating material onto the array and the first surface of the substrate sheet so that each package site is encapsulated; removing the substrate sheet from the array; and separating individual packages from the encapsulated array, wherein a planar first surface of the die pad and leads of the packages is exposed at an external surface of the package. 11.The method of claim 10, further comprising applying a second layer of an electrically conductive material on the die pads and leads so that the second layer overhangs the first layer. 12.The method of claim 11, wherein the substrate sheet is a plastic material. 13.The method of claim 12, wherein the first and second layers of electrically conductive material are applied by silk screening. 14.The method of claim 12, wherein the first and second layers of electrically conductive material are applied using a stencil. 15.A method of forming a package for an integrated circuit device comprising: providing a plastic sheet having a first surface; applying a metal layer to the first surface of the first plastic sheet; patterning the metal layer to form a die pad and a plurality of leads adjacent to the die pad; placing an integrated circuit device on the die pad; consecutively connecting the integrated circuit device to one or more of the leads; applying an encapsulating material so that the integrated circuit device and the first surface of the plastic sheet are covered by the encapsulant material; and removing the plastic sheet, wherein a planar first surface of the die pad and a first surface of the leads are exposed at an exterior surface of the package. 16.The method of claim 15, wherein removing the plastic sheet comprises using a solvent to dissolve a connection between the die pad and leads and the plastic sheet so that the plastic sheet may be separated therefrom. 17.The method of claim 15, wherein applying the first metal layer to the first surface of the plastic sheet comprises: providing a sheet of metal: and attaching the sheet of metal to the first surface of the plastic sheet with an adhesive. 18.The method of claim 15, wherein patterning the metal layer comprises: applying second metal layer in a pattern on the first metal layer, wherein the pattern of the second metal layer defines said package sites; and selectively etching the first metal layer to form said package sites using said second metal layer as a mask. 19.The method of claim 18, wherein the first metal layer comprises copper and the second metal layer is gold. 20.A method of forming a package for an integrated circuit device comprising:providing a substrate sheet having a first surface; applying a first layer of a electrically conductive material on the substrate sheet, wherein said first layer is applied in a pattern that defines an array of package sites, wherein each package site formed includes a die pad and a plurality of leads adjacent to the die pad; placing an integrated circuit device on the die pad; consecutively connecting the integrated circuit device to one or more of the leads; applying an encapsulating material so that the integrated circuit device and the first surface of the substrate sheet are covered by the encapsulant material; and removing the substrate sheet, wherein a planar first surface of the die pad and a first surface of the leads are exposed at an exterior surface of the package. 21.The method of claim 20, further comprising applying a second layer of an electrically conductive material on the die pads and leads so that the second layer overhangs the first layer. 22.The method of claim 21, wherein the substrate sheet is formed of a plastic material, and the plastic sheet is removed by peeling or dissolving the plastic sheet in a solvent. 23.The method of claim 21, wherein the substrate sheet is formed of a plastic material, and an adhesive connection between the plastic sheet and the encapsulated array is dissolved with a solvent. 24.The method of Claim 1, wherein removing the plastic sheet includes subjecting the plastic sheet to ultraviolet light. 